Catalyst and catalytic oxidative dehydrogenation of alkylaromatics and paraffins

ABSTRACT

PCT No. PCT/EP95/02483 Sec. 371 Date Mar. 13, 1997 Sec. 102(e) Date Mar. 13, 1997 PCT Filed Jun. 26, 1995 PCT Pub. No. WO96/01796 PCT Pub. Date Jan. 25, 1996Alkenylaromatics are produced by catalytic oxidative dehydrogenation of alkylaromatics employing a redox catalyst which is bismuth oxide, in combination with an additive compound of an alkali metal and/or an alkaline earth metal, on a titanium diooxide carrier. In a first reaction step, an alkylaromatic starting material is oxidatively dehydrogenated with the redox catalyst in the absence of molecular oxygen with attending reduction of the redox catalyst. In a second reaction step, the reduced redox catalyst is reoxidized with an oxygen-containing gas.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process and a catalyst for thecatalytic oxidative dehydrogenation of alkylaromatics and paraffinhydrocarbons to give the corresponding alkenylaromatics and olefins,preferably of ethylbenzene to give styrene, water being formed and thedehydrogenation taking place in the absence of free oxidizing agent (ie.oxidizing agent added continuously to the stream of starting materials),such as molecular oxygen or oxygen-containing gases and the redoxcatalyst consisting of at least one reducible metal oxide being the soleoxygen source and performing the function of an oxygen store or oxygencarrier.

Olefins and alkenylbenzenes, in particular styrene and divinylbenzene,are important monomers for engineering plastics and are produced inlarge amounts. Styrene is prepared predominantly by nonoxidativedehydrogenation of ethylbenzene over a modified iron oxide catalyst, onemole of hydrogen being formed per mole of styrene. Unfortunately, thisis an equilibrium reaction which is carried out at high temperatures of,typically, from 600 to 700° C. and takes place with the conversion ofabout 60% at a styrene selectivity of about 90%.

The equilibrium can be overcome and a virtually quantitative conversionachieved by oxidative dehydrogenation in which an oxidizing agent, suchas molecular oxygen or an oxygen-containing as, is introduced into thestream of starting materials, since water is now formed as anaccompanying product. Furthermore, lower reaction temperatures arerequired for this reaction. However, the disadvantage of this process isthe loss of selectivity with respect to the desired product owing to thepresence of oxygen, since the high oxygen concentration in the reactionzone promotes total oxidation as a secondary reaction.

It is therefore being proposed to use, as the oxygen carrier, a catalystconsisting of a reducible metal oxide instead of free oxidizing agent(oxygen). The catalyst (simultaneously an oxygen carrier) is graduallyconsumed and must be regenerated in a second step, restoring the initialactivity. In the regeneration phase, for example, coke deposits can alsobe burnt off. The regeneration is highly exothermic so that the wasteheat liberated can be used, for example, for generating steam.

By decoupling the reduction step and oxidation step, the selectivity canbe substantially increased.

Two variants are available for the technical realization of thisproposal, ie. the separation of the two steps in terms of space and interms of time.

In the former, a moving bed or a circulating fluidized bed is used sothat, after the resulting reaction products have been separated off, thecatalyst particles are transported from the dehydrogenation zone to aseparate regeneration reactor, in which the reoxidation is carried out.The regenerated catalyst is recycled to the dehydrogenation zone. Thecatalyst is exposed to high mechanical stresses and must therefore havesufficient hardness.

2. Description of the Related Art

The embodiment using a fixed bed involves periodic switching between thestarting material feed and, if necessary after a flushing phase, theregeneration gas.

This principle of separation of the two steps of the redox reactionusing a reducible and regeneratable catalyst was first described for theoxidation or ammoxidation of propene to acrolein and acrylic acid oracrylonitrile, respectively (GB 885422; GB 999629; K. Aykan, J. Catal.12 (1968), 281-190), arsenate and molybdate catalysts being used. Theuse of the process in the oxidative dehydrogenation of aliphatic alkanesto mono- and diolefins using ferrite catalysts (e.g. U.S. Pat. No.3,440,299, DE 21 18 344, DE 17 93 499) is likewise known, as is the usefor the oxidative coupling of methane to give higher hydrocarbons,different catalyst classes being used (eg. U.S. Pat. No. 4,795,849, DE 3586 769 with Mn/Mg/Si oxides; U.S. Pat. No. 4,568,789 with Ru oxide; EP254 423 with Mn/B oxides on MgO; GB 2 156 842 with Mn₃ O₄ spinels). Thedehydrodimerization of toluene to stilbene in the absence of free oxygenby means of reducible catalysts, such as Bi/In/Ag oxides (EP 30 837) isalso known. Finally, the principle is also applied to thedehydrogenation, dehydrocyclization and dehydroaromatization of paraffinhydrocarbons for gasoline refinement (U.S. Pat. No. 4,396,537 with Co/Poxide catalysts).

EP 397 637 and 403 462 disclose using the principle of the process forthe oxidative dehydrogenation of paraffin hydrocarbons andalkylaromatics. According to these publications, reducible metal oxidesselected from the group consisting of V, Cr, Mn, Fe, Co, Pb, Bi, Mo, Uand Sn are applied to carriers comprising clays, zeolites and oxides ofTi, Zr, Zn, Th, Mg, Ca, Ba, Si and Al and used. V/MgO is particularlypreferred.

Although a high yield can be obtained with these catalysts, verypronounced gasification (total combustion) occurs in the initial phaseof the dehydrogenation when the hydrocarbon comes into contact with thefreshly regenerated and therefore particularly active catalyst. Apartfrom the loss of raw materials, in addition the amount of oxygenconsumed is considerably more than for the straightforwarddehydrogenation, so that a large part of the oxygen carrier isprematurely exhausted and the cycle times unnecessarily become shorter.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an oxygen-carryingcatalyst which permits higher conversions than the non-oxidativedehydrogenation and at the same time substantially avoids thedisadvantageous behavior of the catalysts of the oxidativedehydrogenation in the initial phase, so that the selectivity isincreased and additional process engineering steps, for example partialpreliminary reduction of the oxygen carrier, can be dispensed with. Itis a further object of the present invention to prepare a particularlyhard catalyst which withstands the mechanical stress in industrialreactors without disintegrating.

We have found that these objects are achieved and that reducible metaloxides, such as oxides of Bi, V, Ce, Fe, In, Ag, Cu, Co, Mn, Pb, Sn, Mo,W, As and Sb, preferably Bi, Ce and V oxides, particularly preferablyBi₂ O₃ or CeO₂, on a titanium dioxide carrier can be used asparticularly advantageous oxygen-carrying catalysts for the oxidativedehydrogenation carried out in the absence of free oxidizing agent. Wehave also found that the addition of an alkali metal and/or alkalineearth metal, preferably Li, Na or Cs, in particular K, particularlyeffectively suppresses the initial gasification and permits largeincreases in the selectivity and in the yield of dehydrogenated product.The addition of alkaline earth metals and rare earth metals is alsoeffective, lanthanum being particularly advantageous.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred catalyst contains 3-30, preferably 5-20, % by weight of K₂O, 5-50, preferably 10-30, % by weight of Bi₂ O₃, and TiO₂ in the amountrequired to complete the balance.

Another preferred catalyst contains 3-30, preferably 5-20, % by weightof Cs₂ O, 5-50, preferably 10-30, % by weight of Bi₂ O₃, and TiO₂ in theamount required to complete the balance.

Another advantageous catalyst contains 3-40, preferably 5-30, % byweight of La₂ O₃, 5-50, preferably 10-30, % by weight of Bi₂ O₃, andTiO₂ in the amount required to complete the balance.

A further advantageous catalyst contains 0-30, preferably 5-20, % byweight of Cs₂ O, 0-30, preferably 5-20, % by weight of K₂ O, 0-40,preferably 5-30, % by weight of La₂ O₃, 5-50, preferably 10-30, % byweight of Bi₂ O₃, and TiO₂ in the amount required to complete thebalance. An advantageous effect is also achieved by the joint use of,for example, Cs/La.

The above ratios are based on the prepared catalyst in the most stableor the stated oxidation state in each case. Hence, it is not intended tomake any statement about the actual binding ratios and there is nointention of restricting the invention in this respect; for example,calcination may also result in the formation of phases which correspondto higher oxidation states of chromium, such as chromates or dichromatesof potassium or of bismuth.

The catalyst can be prepared by conventional methods, such as dryblending, suspension, impregnation, precipitation, spray drying, etc.The ingredients may be used, for example, in the form of their oxides,hydroxides, carbonates, acetates, nitrates or generally water-solublesalts with organic or inorganic anions, which are converted into thecorresponding oxides on heating (calcination). For example, transitionmetal complexes may also be used. The calcination is carried out at,typically, above 200° C. (up to 1000° C.), preferably from 200 to 800°C., in particular from 400 to 700° C.

The dehydrogenation reaction requires a temperature of from 200 to 800°C., preferably from 350 to 550° C., and atmospheric or slightly reducedor superatmospheric pressure, for example from 100 mbar to 10 bar,preferably from 500 mbar to 2 bar, at an LHSV of from 0.01 to 20 h⁻¹,preferably from 0.1 to 5 h⁻¹. In addition to the hydrocarbon to behydrogenated, diluents, for example CO₂, N₂, noble gases or steam, maybe present. The regeneration of the reduced catalyst requirestemperatures of from 300 to 900° C., preferably from 400 to 800° C., theoxidizing agent used being, for example, N₂ O or an oxygen-containinggas. Here too, diluents may be present in the reactor feed. Suitableregenerating gases are, for example, air, air having a low oxygencontent or N₂ O mixtures.

The regeneration can be operated at reduced, atmospheric orsuperatmospheric pressure. Pressures of from 500 mbar to 10 bar arepreferred.

When lanthanum is used, La₂ O₃ should not be used as a startingmaterial. Instead, compounds containing organic radicals, preferablylanthanum acetate, should be used, calcination of said compounds leadingto finely divided La₂ O₃ having a large surface area.

EXAMPLE 1

60 g of TiO₂ are dry-blended with 33.22 g of basic bismuth carbonate Bi₂CO₅ (containing 81% by weight of Bi) and 14.67 g of K₂ CO₃, and theprocedure is continued as described below (Comparative Experiment 2).The catalyst contains 10% by weight of K₂ O, 30% by weight of Bi₂ O₃ and60% by weight of TiO₂.

EXAMPLE 2

65 g of TiO₂ are dry-blended with 27.69 g of basic bismuth carbonate Bi₂CO₅ (containing 81% by weight of Bi) and 14.67 g of K₂ CO₃, and theprocedure is continued as described above. The catalyst contains 10% byweight of K₂ O, 25% by weight of Bi₂ O₃ and 65% by weight of TiO₂.

EXAMPLE 3

60 g of TiO₂ are dry-blended with 27.69 g of basic bismuth carbonate Bi₂CO₅ (containing 81% by weight of Bi) and 22.0 g of K₂ CO₃, and theprocedure is continued as described above. The catalyst contains 15% byweight of K₂ O, 25% by weight of Bi₂ O₃ and 60% by weight of TiO₂.

EXAMPLE 4

55 g of TiO₂ are dry-blended with 27.69 g of basic bismuth carbonate Bi₂CO₅ (containing 81% by weight of Bi) and 29.34 g of K₂ CO₃, and theprocedure is continued as described above. The catalyst contains 20% byweight of K₂ O, 25% by weight of Bi₂ O₃ and 55% by weight of TiO₂.

EXAMPLE 5

60 g of TiO₂ are dry-blended with 33.2 g of basic bismuth carbonate Bi₂CO₅ (containing 81% by weight of Bi) and 11.65 g of Cs₂ CO₃, and theprocedure is continued as described above. The catalyst contains 10% byweight of Cs₂ O, 30% by weight of Bi₂ O₃ and 60% by weight of TiO₂.

EXAMPLE 6

60 g of TiO₂ are dry-blended with 27.69 g of basic bismuth carbonate Bi₂CO₅ (containing 81% by weight of Bi) and 17.34 g of Cs₂ CO₃, and theprocedure is continued as described above. The catalyst contains 15% byweight of Cs₂ O, 25% by weight of Bi₂ O₃ and 60% by weight of TiO₂.

EXAMPLE 7

55 g of TiO₂ are dry-blended with 27.69 g of basic bismuth carbonate Bi₂CO₅ (containing 81% by weight of Bi) and 14.67 g of K₂ CO₃ and 11.56 gof Cs₂ CO₃, and the procedure is continued as described above. Thecatalyst contains 10% by weight of Cs₂ O, 10% by weight of K₂ O, 25% byweight of Bi₂ O₃ and 55% by weight of TiO₂.

EXAMPLE 8

75 g of TiO₂ are dry-blended with 83.05 g of basic bismuth carbonate Bi₂CO₅ (containing 81% by weight of Bi), 36.68 g of K₂ CO₃ and 161.53 g oflanthanum acetate C₆ H₉ LaO₆.aq (containing 39.56% by weight of La), andthe procedure is continued as described above. The catalyst contains 10%by weight of K₂ O, 30% by weight of La₂ O₃, 30% by weight of Bi₂ O₃ and30% by weight of TiO₂.

COMPARATIVE EXPERIMENT 1 (according to EP 397637)

100 g of SiO₂ powder D11-10 from BASF are calcined at 500° C. for 3hours. A solution of 102.5 g of Fe(NO₃)₃.9H₂ O in demineralized water isadded to 85 g of the calcined SiO₂ until a final weight of 216 g isreached. The mixture is left to stand for 18 hours at room temperature,filtered and dried at 110° C. for 18 hours. Thereafter, it is heated ata rate of 30° C./h to 600° C. and calcined for 10 hours at 600° C. Thecatalyst is in powder form and contains 20% by weight of Fe₂ O₃ and 80%by weight of SiO₂.

COMPARATIVE EXPERIMENT 2 (according to EP 397637)

336.3 g of ammonium metavanadate and 900 g of magnesium oxide arestirred into 8 l of water and then stirred vigorously for 1 hour. Themixture is then spray-dried. The spray-dried powder obtained is treatedin a kneader for 2 hours, a little water and extrusion assistant beingkneaded into the material. The kneaded material is then extruded to give3 mm solid extrudates. The extrudates are dried for 16 hours at 120° C.and then calcined for 4 hours at 600° C. Uniformly yellow extrudateshaving low hardness are obtained. For the reactor experiments, a0.05-0.1 mm chip fraction is separated off by sieving. The catalystcontains 22.5% by weight of V₂ O₅ and 77.5% by weight of MgO.

COMPARATIVE EXPERIMENT 3

100 g of MgO powder and 58.1 g of NH₄ VO₃ powder are dry-blended for 1hour. The mixture is then kneaded in a kneader for 2.5 hours.Thereafter, the kneaded material is extruded to give 3 mm solidextrudates. The extrudates are dried for 2 hours at 120° C. and thencalcined for 2 hours at 500° C. For the reactor experiments, a 0.05-0.1mm chip fraction is separated off by sieving. XRD shows only the linesof V₂ O₅ and MgO. The catalyst contains 31% by weight of V₂ O₅ and 69%by weight of MgO.

COMPARATIVE EXPERIMENT 4

100 g of TiO₂ powder (DT-51 from BASF AG) are initially taken, and theprocedure is continued according to Comparative Experiment 2. Thecatalyst consists of pure TiO₂.

COMPARATIVE EXPERIMENT 5

276.85 g of basic bismuth carbonate are initially taken and theprocedure is continued according to the comparative experiment. Thecatalyst consists of pure Bi₂ O₃.

COMPARATIVE EXPERIMENT 6

200 g of TiO₂ are dry-blended with 129.2 g of basic bismuth carbonateBi₂ CO₅ (containing 81% by weight of Bi), and the procedure is continuedaccording to Comparative Experiment 2. The catalyst contains 63% byweight of TiO₂ and 37% by weight of Bi₂ O₃.

COMPARATIVE EXPERIMENT 7

200 g of TiO₂ are dry-blended with 58.6 g of NH₄ VO₃, and the procedureis continued according to Comparative Experiment 2. The catalystcontains 19% by weight of V₂ O₅ and 81% by weight of TiO₂.

The hardness of the catalysts with respect to cutting is determinedusing a 3 mm solid extrudate, measuring the force in N required to cutthrough the extrudate with a sharp knife (blade width 0.6 mm).

The catalytic oxidative dehydrogenation of ethylbenzene to styrene iscarried out in a pulsed reactor at 500° C. A pulsed stream of pureethylbenzene is passed through a microfixed bed (catalyst weight: 0.3g), and the resulting reaction products are determined quantitatively bygas chromatography for each pulse. Between two successive ethylbenzenepulses (about 1.5 minutes), helium flows through the reactor. Anindividual pulse contains 380 μg of ethylbenzene. The flow rate of thecarrier gas is 21.5 ml/min. In this way, the behavior of the catalyst asa function of time can be monitored without dead times from thebeginning with high time resolution.

At the beginning of the reaction, the catalyst is highly active, so thathigh, virtually quantitative conversions of ethylbenzene are observed.In the further course of the reaction, the selectivity with respect tostyrene improves steadily until a final value is reached. However, withprogressing duration of the experiment, the catalyst is increasinglydeactivated at the rate at which its oxygen content is consumed, so thatthe conversion decreases. Regeneration is carried out after from 90 to200 pulses, depending on the catalyst. The styrene yield as the productof selectivity and conversion generally passes through a gentle maximum.The yield listed in the table is based on this maximum value.

After the end of the dehydrogenation reaction, the feed is changed overto an air stream of 25 ml/min, and the catalyst is regenerated for about1 hour at 500° C. This is followed by the next cycle. A plurality ofcycles are investigated in each case.

The results of the examples/experiments are shown in the table below.The table contains a summary of the catalysts prepared, of the relativeratios of the components, of the hardness with respect to cutting and ofthe results of the tests (mean values from several experiments) for thedehydrogenation of ethylbenzene in a fixed-bed reactor.

The following conclusions may be drawn from the table:

The prior art systems are very active and permit a high maximum styreneyield. The decisive disadvantage is the pronounced initial gasification,which leads to enormous losses of ethylbenzene and depletes the oxygenreservoir of the catalyst. In particular, the initial ethylbenzenepulses are completely combusted (100% gasification to useless carbondioxide), so that the theoretical initial selectivity with respect tostyrene is zero for the initial pulses.

In contrast, the novel systems exhibit a substantially lower level ofgasification while likewise having very high activity. Thus, the initialgasification (1st pulse) is only 30% by weight, compared with 100% byweight according to the prior art.

The styrene yield is substantially above that which can be obtained bymeans of nonoxidative dehydrogenation, this being the case at a lowerreaction temperature.

As shown in Comparative Experiments 1 and 2, the preparation method isimportant in the case of the known catalysts. It is evident that theknown spray-dried catalyst is substantially better than the dry-blendedone.

However, the spray drying step in the catalyst preparation is atime-consuming and energy-intensive step which entails relatively highproduction costs. In the spray drying of MgO, the amount of water whichmust be added per gram of solid is higher than in the case of TiO₂, sothat this process step takes place more rapidly with TiO₂ than with MgO.

The styrene selectivity of the novel catalyst at the maximum iscomparable with the prior art, the catalyst preparation by dry-blendingrequiring substantially less time and substantially less expensiveapparatus.

Although the novel catalyst produces slightly larger amounts of thebiproducts benzene and toluene in the initial phase (instead of gas, asin the prior art), these amounts then rapidly decrease in the course ofthe reaction. The formation of benzene and toluene presents no problemscompared with CO₂ formation, since toluene is a saleable product andbenzene can be recycled to the ethylbenzene preparation and henceneither is lost. The novel catalyst is therefore also superior to theprior art in this respect, regardless of the fact that the averagestyrene yield or the total styrene yield is more advantageous than inthe case of the known catalysts.

The decisive advantage of the novel system is the substantially reducedinitial gasification, which permits enormous gains in the initialstyrene selectivity compared with the prior art.

The novel catalyst is furthermore particularly abrasion-resistant. Thishas advantages for the mechanical handling of the catalysts (transport,installation in and removal from the reactor) and with regard to themechanical load to which the catalysts are subjected in the fixed bed,and it must also be taken into account that the reoxidation results inthe liberation of considerable quantities of heat which subject thecatalyst to considerable mechanical stress. With the same preparationmethod, the novel catalyst (Examples 1 to 7) has substantially bettermechanical strength compared with the Comparative Experiments 1 and 2.

                                      TABLE    __________________________________________________________________________                      Amounts by  Conversion                                        Selectivity                                              Yield                      weight                            Cutting                                   % by wt!                                         % by wt!                                               % by wt!    Example           Catalyst    % by wt!                            hardness N                                  (at styrene maximum)    __________________________________________________________________________    1      Bi.sub.2 O.sub.3 /TiO.sub.2                      37:63 34    42    81    34    2      V.sub.2 O.sub.5 /TiO.sub.2                      19:81 15    43    95    40    3      K.sub.2 O/Bi.sub.2 O.sub.3 /TiO.sub.2                      10:30:60                            41    75    92    69                                  96    88    84    4      K.sub.2 O/Bi.sub.2 O.sub.3 /Ti.sub.2                      10:25:65                            33    94    90    84    5      K.sub.2 O/Bi.sub.2 O.sub.3 /TiO.sub.2                      15:25:60                            38    97    93    90    6      K.sub.2 O/Bi.sub.2 O.sub.3 /TiO.sub.2                      20:25:55                            22    96    92    89    7      Cs.sub.2 O/Bi.sub.2 O.sub.3 /TiO.sub.2                      10:30:60                            39    89    89    79                                  96    85    82    8      Cs.sub.2 O/Bi.sub.2 O.sub.3 /TiO.sub.2                      15:25:60                            34    94    89    84    9      K.sub.2 O/Cs.sub.2 O/Bi.sub.2 O.sub.3 /TiO.sub.2                      10:10:25:55                            30    98    92    89    10     K.sub.2 O/La.sub.2 O.sub.3 /Bi.sub.2 O.sub.3 /TiO.sub.2                      10:30:30:30                             8    95    91    86                                  99    93    92    Comparison 1           V.sub.2 O.sub.5 /MgO                      22:78  0    99    94    93    Comparison 2           2 V.sub.2 O.sub.5 /MgO                      31:69 22    92    89    82    Comparison 3           TiO.sub.2 DT-51                      100   22    22    68    15    Comparison 4           Bi.sub.2 O.sub.3                      100    4    B8           6    __________________________________________________________________________

    ______________________________________                         Initial gasification             Initial selectivity                          % by weight!                                      Residence time    Example   % by weight!                         1st/10th/20th pulse                                       sec!    ______________________________________    1         2/39/38    96/30/38     0.2    2         0/42/67    100/23/5     0.2    3        69/89/92    23/5/3       0.2             51/84/88    31/7/6       0.4    4        49/82/86    35/10/7      0.4    5        53/87/93    31/4/3       0.4    6        53/90/92    32/3/2       0.4    7        71/89/90    23/7/5       0.2             48/75/79    36/15/12     0.4    8        43/78/63    45/13/10     0.4    9        55/89/92    30/3/2       0.4    10       57/88/90    30/4/3       0.2             51/82/85    31/5/4       0.4    Comparison 1              0/52/76    100/30/15    0.2    Comparison 2              0/39/61    100/45/22    0.2    Comparison 3              3/68/75    96/4/3       0.4    Comparison 4         100/6/5      0.2    ______________________________________

We claim:
 1. A process for the catalytic oxidative dehydrogenation of analkylaromatic to yield the corresponding alkenylaromatic, which processcomprises: providing a redox catalyst, which serves as an oxygencarrier, the redox catalyst being prepared by admixing catalyticcomponent bismuth oxide and a titanium oxide carrier in the presence oflanthanum and an additional catalytic component comprising a compound ofa number of the group consisting of alkali metals and alkaline earthmetals;oxidatively dehydrogenating the alkylaromatic with the redoxcatalyst to produce a corresponding alkenylaromatic in a first reactionstep, the redox catalyst being reduced in this step; and reoxidizing thereduced redox catalyst with an oxygen-containing gas in a secondreaction step.
 2. A process as claimed in claim 1, wherein the catalystcontains an inorganic binder.
 3. A process as claimed in claim 1,wherein the first and the second reaction steps take place alternatelyin terms of time or at alternate places and the catalyst is contained ina fixed bed reactor.
 4. A process as claimed in claim 3, whereindecoupling of the steps is effected by periodically switching thereactor inlet stream between starting material and oxidizing agent.
 5. Aprocess as claimed in claim 4, wherein the spatial decoupling of thesteps is effected with the use of a circulating fluidized bed, bycirculating catalyst particles cyclically between a dehydrogenationreactor and a regeneration reactor.
 6. A process as claimed in claim 3,wherein the catalyst is contained in a fixed bed reactor and a flushingphase, in which a flushing gas flows through the fixed bed reactor, isintroduced between the steps.
 7. A process as claimed in claim 6,wherein the flushing gas used is CO₂, N₂, H₂ O or a noble gas.
 8. Aprocess as claimed in claim 1, wherein ethylbenzene is dehydrogenated tostyrene.
 9. A process as claimed in claim 1, wherein the dehydrogenationis carried out at from 200 to 800° C. and at from 100 mbar to 10 bar ata liquid hourly space velocity (LHSV) of from 0.01 to 20 h⁻¹.
 10. Aprocess as defined in claim 1, wherein the redox catalyst comprises from5 to 50% by weight of bismuth (3) oxide and from 3 to 30% by weight ofK₂ O or Cs₂ O, the remainder being titanium oxide and lanthanum, withthe proviso that the sum of the percentages by weight is
 100. 11. Theprocess of claim 10, wherein the redox catalyst additionally comprises 5to 30% by weight of lanthanum oxide, with the proviso that the sum ofthe percentages by weight is 100.